1. Field of the Invention
The invention relates to an image heating apparatus suitable for use as a heat-fixing device carried on an image forming apparatus such as a copying machine or a printer, and particularly to an image heating apparatus having a flexible metallic sleeve.
2. Description of Related Art
Heat-fixing apparatuses of a heat roller type or a film heating type have heretofore been widely used as fixing apparatuses applied to image forming apparatuses such as copying machines or printers. Particularly, a method whereby during standby, electric power is not supplied to a heat-fixing apparatus to thereby minimize the consumption of electric power, and more particularly a heat-fixing method using a film heating process of fixing a toner image on a recording material with film interposed between a heater portion and a pressure roller are proposed in Japanese Patent Application Laid-Open No. S63-313182, Japanese Patent Application Laid-Open No. H2-157878, Japanese Patent Application Laid-Open No. H4-44075, Japanese Patent Application Laid-Open No. H4-204980, etc. As the construction of a film heat-fixing device, there are method of using a transport roller exclusively for the transport of film and a driver roller to transport the film between the transport roller and a pressure roller while applying tension to the film, and a method of driving cylindrical film by a transporting force from a pressure roller, and the former method has a merit of being capable of keeping film transporting performance high, and the latter method has a merit of being capable of realizing a low-cost fixing device resulting from the simplification of construction.
As a specific example, the cross-sectional construction of a heat-fixing device of the latter pressure roller driving type is schematically shown in FIGS. 2, 3 and 5 of the accompanying drawings. FIGS. 2, 3, and 5 show portions common to those of a fixing apparatus to which the present invention is applied and which will be described later. The fixing apparatus 10 shown in FIGS. 2 and 3 has a heating member (hereinafter referred to as the heater holder) 11 fixedly supported by a holding member (hereinafter referred to as the heater holder) 12, heat-resistant thin film (hereinafter referred to as the fixing film) 13 rotated while being in contact with the heater 11, and an elastic pressure roller 20 brought into pressure contact with the heater 11 with a nip part (fixing nip part) N of a predetermined nip width formed with the fixing film 13 interposed therebetween. The heater holding portion of the heater holder 12 is longer than the lengthwise direction (a direction perpendicular to the plane of the drawing sheet of FIG. 2) of the fixing film 13, and protrudes from the opposite end portions of the fixing film 13. This protruding portion is biased toward the pressure roller 20 side by a spring, not shown. The heater holder 12 is formed by a heat-resistant molded member or the like and produces flexure by being pressurized and therefore, a reinforcing member 30 is made to abut against the counter-heating member side of the heater holder 12 to thereby prevent the flexure. The heater 11 is heated and controlled to a predetermined temperature by being electrically energized. The fixing film 13 is a cylindrical thin member transported in the direction of arrow by a rotative driving force from driving means, not shown, or the pressure roller 20 while being in close contact with and sliding relative to the surface of the heater at the fixing nip part N.
When in a state in which the heater 11 is heated and controlled to the predetermined temperature and the fixing film 13 has been transported in the direction of arrow, a recording material P having an unfixed toner image T formed and borne thereon is introduced into between the fixing film 13 and the pressure roller 20 at the fixing nip part N, the recording material P is in close contact with the surface of the fixing film 13 and is nipped and transported with the fixing film 13 by the fixing nip part N. At this fixing nip part N, the toner image T on the recording material P is heated through the fixing film 13 heated by the heater 11, whereby it is fixed as a permanent image on the recording material P. The recording material P passed through the fixing nip part N is stripped off from the surface of the fixing film 13 and is transported.
A ceramic heater is generally used as the heater 11 as a heating member. This heater will hereinafter be described in detail with reference to FIG. 3.
For example, an energized heat-generating resistor layer 11b of silver palladium (Ag/Pb).Ta2N or the like is formed on the surface of a ceramic substrate 11a of good electrical insulativeness, good thermal conductivity and low heat capacity such as alumina (a surface on that side thereof which faces the fixing film 13) along the lengthwise direction of the substrate (a direction orthogonal to the transport direction of the recording material, and a direction perpendicular to the plane of the drawing sheet of FIG. 3) by screen printing or the like, and further the surface on which the heat-generating resistor layer is formed is covered with a thin glass protective layer 11c. This ceramic heater 11 is such that the energized heat-generating resistor layer 11b is electrically energized to thereby generate heat and the entire heater comprising the ceramic substrate 11a and the glass protective layer 11c rapidly rises in temperature. This temperature rise of the heater 11 is detected by a temperature detecting element 14 disposed on the back of the heater and is fed back to an energization controlling portion, not shown. The energization controlling portion controls the electrical energization of the energized heat-generating resistor layer 11b so that the temperature of the heater detected by the temperature detecting element 14 may be maintained at a predetermined substantially constant temperature (fixing temperature). That is, the heater 11 is heated and controlled to a predetermined fixing temperature.
The fixing film 13 has its thickness made as small as 20-70 μm in order to efficiently give the heat from the heater 11 to the recording material P at the fixing nip part N. This fixing film 13 is constituted by three layers, i.e., a film base layer a primer layer and a mold-releasable layer, and the film base layer side thereof is a heater side and the mold-releasable layer side thereof is a pressure roller side. The film base layer is formed of polyimide, polyamideimide, PEEK or the like which is higher in insulativeness than the glass protective layer, and has heat resistance and high elasticity. Also, the mechanical strength such as the tear strength of the entire fixing film is kept by the film base layer. The primer layer is a thin layer having a thickness of the order of 2-6 μm. The mold-releasable layer is a toner offset preventing layer for the fixing film, and is covered to a thickness of the order of 10 μm with fluorine resin such as PFA, PTFE or FEP.
Also, the heater holder 12 is formed, for example, by a heat-resistant plastic member, and holds the heater 11 and serves also as a transport guide for the fixing film 13. The reinforcing member 30 is formed of a metal material in order not to produce the flexure of the heater holder by a pressure force, and the cross-sectional shape thereof is an “inverted U-shape” shown in FIG. 4A of the accompanying drawings, or a “U-shape” shown in FIG. 4B of the accompanying drawings.
In a heating apparatus of a film heating type using such thin fixing film, the pressure roller 20 having an elastic layer 22 is brought into pressure contact with the flattened underside of the heater 11 because of the high rigidity of the ceramic heater 11 as a heating member, whereby the fixing nip part N of a predetermined width is formed, and only the fixing nip part N is heated to thereby realize heat-fixing of quick start.
In the above-described construction, the arrangement relationship between the energized heat-generating resistor layer of the heater 11 and the pressure roller 20 will now be described with reference to FIG. 5.
In FIG. 5, the lengthwise width W of the energized heat-generating resistor layer 11b of the heater 11 is somewhat narrow as compared with the width D of the elastic layer 22 of the pressure roller 20 brought into pressure contact with the heater with the fixing film 13 interposed therebetween. This is for preventing the energized heat-generating resistor layer 11b from jutting out from the pressure roller 20 to thereby locally raise the temperature of the heater 11 and damage the heater 11 by the thermal stress thereof. Also, the energized heat-generating resistor layer 11b is formed with a width sufficiently wider than a transport area for the recording material P having the toner image formed and borne thereon. Thereby, the influence of the temperature drop of the end portion (due to the leakage of heat to electrical contacts for energization and connectors at the end portions of the heater) can be eliminated, whereby a good fixing property is obtained over the entire surface of the recording material. Further, there is a case where the width of the energized heat-generating resistor layer at the end portions of a sheet passing area is reduced and the amount of generated heat at the end portions is increased to thereby make up for the fixing property of the end portions.
Thereby, the heat generated by electrically energizing the energized heat-generating resistor layer 11b of the heater 11 is efficiently given to the recording material P transported between the fixing film 13 and the pressure roller 20 to thereby act to fuse and fix the toner image T on the recording material P.
Also, the letter S designates a recording material transport standard, and in this case, it designates a central standard device having a standard provided at the lengthwisely center of the recording material transport area of an image forming apparatus main body.
Further, as shown in FIG. 5, the temperature detecting element 14 such as a thermistor and a thermoprotector 15 such as a temperature fuse or a thermoswitch which is a safety element for shutting down the electrical energization of the energized heat-generating resistor layer 11b of the heater 11 during wild run abut against the back of the heater, and these are disposed in a transport area for a recording material of a minimum width transportable by the image forming apparatus. The temperature detecting element 14 and the thermoprotector 15 are designed to be contained in the interior of the metallic reinforcing member 30.
The temperature detecting element 14 is provided in a transport area for a recording material of a usable minimum definite size in order to heat and fix a toner image on the recording material at a moderate fixing temperature without causing such problems as faulty fixing and high temperature offset even when a recording material of a minimum width transportable by the image forming apparatus main body is transported. On the other hand, if the thermoprotector 15 is disposed in a non-transporting area for the recording material when a recording material of a small size is transported, the thermoprotector 15 will malfunction due to the excessive temperature rise of the non-transporting area even during normal transport and will shut out electrical energization and therefore, the thermoprotector 15 is also provided in the transporting area for the recording material of the usable minimum definite size. Also, the thermo protector 15 is made to abut against the back of the heater, whereby it may happen that the amount of heat generated by the energized heat-generating resistor layer 11b is taken away by the thermoprotector 15 and a sufficient amount of heat becomes incapable of being given to the recording material P and faulty fixing is caused at the abutting position of the thermoprotector. In order to prevent this, at a position on the energized heat-generating resistor layer 11b which corresponds to the abutment of the thermoprotector, the width of a portion of the energized heat-generating resistor layer 11b of the heater 11 is somewhat narrowed as shown in FIG. 5 and the resistance value at this abutting position is made greater than that of the other portions to thereby secure an amount of generated heat. Thereby, the amount of heat supplied to the recording material P is made constant in the lengthwise direction to thereby realize good heating and fixing free of the unevenness of fixing. The temperature detecting element 14 is likewise made to abut against the back of the heater and therefore, it is feared that the heat generated by the energized heat-generating resistor layer 11b is likewise taken away by the temperature detecting element 14, but the amount of heat taken away from the heater can be suppressed to a small amount by using a temperature detecting element of a small heat capacity such as a chip thermistor. Thus, even is the above-described countermeasure similar to that for the thermoprotector 15 is not adopted, uniform fixing becomes possible without spoiling the uniformity of the fixing of the recording material in the lengthwise direction.
The heat-fixing apparatus of the film heating type described hitherto does not require preliminary heating during standby due to the high heating efficiency and the possibility of quick start and therefore, enjoys many merits such as the possibility of achieving the saving of electric power and a merit to the user by the elimination of a waiting time, and particularly, a method of driving the cylindrical film by the transporting force of the pressure roller can realize a low cost and therefore it is expected to be introduced into a compact low-speed machine to a large high-speed machine in the future.
To achieve this higher speed, thermal energy sufficient for fixing must be supplied even in the case of a recording material which has become shorter in the time required to pass through the fixing nip part. As means for realizing this, it is conceivable to set the fixing temperature to still a higher temperature, to increase the pressure force between the pressure roller and the fixing film and widen the width of the fixing nip which is a heating area, or to change the materials of the heater substrate and the fixing film to ones excellent in thermal conductivity to thereby increase the amount of supplied heat.
However, such an improvement, if carried out, will increase the load to the fixing film and promote the deterioration of the fixing film, and this will lead to the disadvantage that service life becomes short.
For example, if in order to improve the thermal conductivity of the base layer of the fixing film, the amount of addition of a filter of high thermal conductivity such as boron nitride (BN) or aluminum nitride (ALN) is increased to thereby contrive an improvement in thermal conductivity, the original flexibility and strength of resin such as polyimide (PI) will be spoiled to thereby hasten the wear and deterioration of the fixing film.
So, what has been newly proposed is to employ as the base material of the fixing film a cylindrical thin-walled rotary member (metallic sleeve) formed of a metal more excellent in thermal conductivity than resin. This metallic sleeve can transmit thermal energy sufficient for fixing to the recording material by the thermal conductivity of the material thereof even if the fixing temperature is not set to a high temperature or the pressure force is not made great in order to make the width of the fixing nip great, and it becomes possible to achieve a film heat-fixing apparatus more excellent in the capability of coping with a high speed.
However, it has been found that when in the heat-fixing apparatus of good thermal efficiency using the metallic sleeve as the fixing film 13, the smaller diameter of the metallic sleeve is contrived for the purposes of making the radiation from the metallic sleeve small, and making the heat capacity of the fixing apparatus 10 comprising the metallic sleeve or the like small, in order to achieve higher thermal efficiency, there arises such a problem as will be described below.
When the smaller diameter of the metallic sleeve is contrived, the distance thereof from the metallic reinforcing member 30 installed on the back of the heater holder 12 becomes smaller. Thereupon, due to the excellent radiative property which is the characteristic of the metal which is the material of the metallic sleeve, thermal energy accumulated in the metallic sleeve is transmitted through the air which is an adiabatic layer and is used to cause the metallic reinforcing member 30 to rise in temperature, and as the result, the surface temperature of the metallic sleeve lowers and the temperature of the reinforcing member 30 rises, and it has been found that when continuous image fixing is effected, the difference between the temperature of the metallic sleeve and the temperature of the rein forcing member becomes as small as the order of several ° C.
Thus, the surface temperature of the metallic sleeve becomes incapable of keeping a temperature necessary to fix an unfixed toner image, and a phenomenon of the fixing property being spoiled occurs.